I notice most of the characters are multi-classed. What's the reason behind that?
Well, its really different reasons. First of, these characters were created back when 4e just came out, so people wanted to explore the different possibilities a bit. But also (a big part of it) was bringing the party together and creating some consistency. After a near-wipe around level 2, I brought out the idea of creating a party that had more in common. Or at least some of them.
Carric was going to a cleric of Kord, and so the Fighter decided to multi-class into cleric (of Kord) as well, to bring them closer together (I am guessing that he didn't mind the extra Healing Word). He was also at the time afraid that a fighter might turn out to be a bit too boring in the long run (they had in the previous editions - to him anyway), so he wanted to keep his options open, maybe going for some ritual casting later on. As it turns out, it was never needed.
The wizard made a background about his father working for the Church of Kord, having visions and premonitions that he expressed as paintings (yeah, and he has never seen an episode of Heroes). That way, we had a trio of the party, the core being Kord-centric. Then, as they gained a few levels, I started giving visions to the wizard. From there, it was a no-brainer to pick up a multi-class feat and multi-class him into cleric (of Kord of course) so that he could pick up divine oracle (and later sage of ages) to round of his character.
Why the Cleric chose to multi-class into ranger and the warlock into bard? The cleric did it to get perception trained, and he felt it fit the theme of the campaign (lots of exploring the new world vibe). The warlock did it for some sort of optimization. Not anything broken or even remotely, but in the mind of his player, it gave him some nifty bonuses that he wanted for the character. This doesn't bother me much. I prefer that people have the character that they want, I have found it makes for much better campaigns.